overview of chapter 21 of asce 7-10
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Site-Specific Risk-Targeted Ground Motion Procedures
Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCECarlsbad, California
consulting engineers and scientists
AEG Inland Empire Chapter Continuing Education SeriesMay 31, 2014
• Overview • Site-specific procedures• Risk coefficient• NGA Relationships• Deaggregation• Examples• Performance Based EE• Summary
Outline
Source, Path and Site
Evaluating Seismic Hazard and Ground Motions
• 2103 CBC, 1616.10.2, 1616A.1.3 “For buildings assigned to Seismic Design Category E and F, or when required by the building official, a ground motion hazard analysis shall be performed in accordance with ASCE 7 Chapter 21, as modified by Section 1803A.6 of this code.”
SITE-SPECIFIC STUDY
• Structures on Site Class F sites (Ts > 0.5 seconds)• At least 5 recorded or simulated horizontal ground motion
acceleration time histories (MCER spectrum at bedrock)
Site Response Analysis
Seismic Hazard Analysis• Seismically isolated structures (S1 0.6)• Structures with damping systems (S1 0.6)• A time history response analysis of the building is performed
(ASCE 7-10, Section 11.4.7, p.67)
SITE-SPECIFIC STUDY (cont’d)
SITE RESPONSE ANALYSIS
(ASCE 7-10, Section 21.1, p.207)
GroundSurface
Rockbase
SITE-SPECIFIC GROUND MOTION PROCEDURE
(Sections 21.2, 21.3, and 21.4)
• Probabilistic ground motion• Method 1: Uniform-hazard GM * Risk Coefficient• Method 2: Risk-targeted probabilistic GM directly
• Deterministic ground motion• 84th-%ile GM, but not < 1.5Fa or 0.6*Fv/T
• MCER = Min (Prob. GM, Det. GM)• All GMs are max-direction spectral accelerations (Sa)
• Risk coefficient: CR
• T ≤ 0.2 s; CR = CRS (Figure 22-17)• T ≥ 1.0 s; CR = CR1 (Figure 22-18)• 0.2 s ≤ T ≤ 1.0 s; CR linear interpolation
of CRS and CR1
Risk coefficient
Risk Coefficient
SITE-SPECIFIC GROUND MOTION PROCEDURE
Prob MCER Det MCER
MCER Spectrum
DESIGN Spectrum
SITE-SPECIFIC DESIGN SPECTRUM
General DESIGN Spectrum
General MCE Spectrum
Site Coord Site Class
1% Prob. of collapse 50 yr(direction of max horiz resp)
Lesser of PSHA and DSHA
2/3 MCE Spectrum2/3 MCE Spectrum
> 80% General Design Spectrum
84th percentile(direction of max horiz resp)
(Sections 21.2, 21.3, and 21.4)
SITE-SPECIFIC GROUND MOTION PROCEDURE
Deterministic Lower Limit (DLL) on MCER Spectrum
1.5 Fa
0.6 Fa
Sa = 0.6 Fv/T
0.08 Fv/Fa 0.4 Fv/Fa TL
Period (seconds)
Sa (g)
Sa = 0.6 Fv TL/T2
(Section 21.2.2, p. 209)
Attenuation Relationships
Several types of ground motions parameters can be calculated from a recorded EQ time history.
But what do you do if you want to estimate what the ground motion parameters are going to be from an earthquake that hasn’t happened yet?
Attenuation Relationships
ANSWER:Use the data that we’ve collected so far and fit equations to them for predicting future ground motions.These equations are often called attenuation relationships.
Attenuation Relationships
Distance from Source
Grou
nd M
otio
n Pa
ram
eter
Initial relationships were just based on Magnitude (M) and Distance (R), but equations become much more complex as researchers looked for ways to minimize data
scatter.
Attenuation Relationships
Modern attenuation relationships have terms that deal with such complexities as:
1) Fault type
2) Fault geometry
3) Hanging wall/Foot wall
4) Site response effects
5) Basin effects
6) Main shock vs. After shock effects
Pretty complex …. Hard to do by hand!!
Attenuation Relationships
Ideally, every geographic area that experiences EQs would have its own set of attenuation relationships. WHY?
Not enough recorded data!
Scatter in the data could be minimized!…But we can’t really produce site-specific
attenuation relationships for places other than those that have a lot of frequent earthquakes. WHY?
So we start combining earthquake records from geographically different areas with the assumption that the ground motions should be similar despite the differences in location. Ergodic Assumption
Three NGA projects:• For active crustal Eqs (California, Middle
East, Japan, Taiwan,…): NGA-West• For subduction Eqs (US Pacific Northwest
and northern California, Japan, Chile, Peru,…): NGA-Sub
• Stable continental regions (Central and Eastern US, portion of Europe, South Africa,…): NGA-East
NGA=Next Generation “Attenuation” Relations
Attenuation Relationships (GMPEs)
For crustal faults in the Western US and other high- to moderate- seismicity areas, most professionals currently use:Next Generation Attenuation Relationships (NGAs)NGA West 1: 5 separate research teams were given the same set of ground motion data and were asked to develop relationships to fit the data. Their results were published in 2008.
-Abrahamson & Silva -Chiou & Youngs-Campbell & Bozorgnia
-Idriss-Boore & Atkinson
(rock only)
NGA-West 1: 2008NGA-West 2: 2014
NGA-West
Data set No. EQs No. Rec Sa Type Damping(%)
Periods (sec)
NGA-West 1
173 3,551 AR, GMRotI50
5 0.01 - 10
NGA-West 2
610 21,331 AR, RotDnn 0.5 - 30 0.01 - 20
AR= as-recorded
Rotate horizontal components, at each period compute:• RotD50 = 50 percentile• RotD100 = max• RotD00 = min
RotDnn
RotD50 is the main intensity measure PGA, PGV and Sa at 21 periods: 0.01, 0.02,……,5, 7.5, 10 sec No GMPE for PGD
• Applicable magnitude range:– M ≤ 8.5 for strike-slip (SS)– M ≤ 8.0 for reverse (RV)– M ≤ 7.5 for normal faults (NM)
• Applicable distance range:– 0 – 200 km (preferably 300km)
NGA West-2 ranges of applicability
Parameter AS BSSA CB CY IMagnitude Mw Mw Mw Mw Mw
Top of rupture Ztor Ztor Ztor
Style of faulting RV, NM, SS RV, NM, SS RV, NM, SS RV, NM, SS RV, NM, SSDip Yes Yes YesDowndip fault width Yes YesClosest distance to rupture
Rrup Rrup Rrup Rrup
Hor. dist. to surface proj. Rjb Rjb Rjb Rjb
Hor. dist. Perpendicular to strike
Rx, Ry Rx Rx
Hanging wall model Yes (Rjb) Yes YesVs30 Vs30 (760m/s) Vs30, (Sj) Vs30 Vs30≥450Depth to Vs Z1.0 Z2.5 Z1.0
Hypocentral depth Hhyp
Vs30 for reference rock (m/s)
1,100 760 1,100 1,130
Horizontal NGA-West 2 GMPEs parameters
• Abrahamson-Silva-Kamai (ASK)• Boore-Stewart-Seyhan-Atkinson (BSSA)• Campbell-Bozorgnia (CB)• Chiou-Youngs (CY)• Idriss (I)
NGA West 2 Five models
NGA Distance Notations
𝑅𝑅𝑢𝑝= Closest distance to rupturing fault plane𝑅 𝐽𝐵= Boore − Joyner distance
𝑅𝑋= Closest horizontal distance to the top of rupture
More on distances
• Geotechnical Services Design Manual, Version 1.0, 2009, Caltrans
• Development of the Caltrans Deterministic PGA Map and Caltrans ARS Online, 2009, Caltrans
NGA Soil vs. Rock
NGA equations don’t have a “trigger” for soil or rock. They just rely on the VS30, which is the average shear wave velocity in the upper 30 meters of the ground.
VS30 (m/s) Type Site Class>150
0760-1500360-760180-360<180
Hard RockFirm RockSoft RockRegular SoilSoft Soil
ABCDE
NGA West 2 Excel spreadsheet
http://peer.berkeley.edu/ngawest2/databases/
2013 CBC, Section 1803A.6 Geohazard Reports
The three Next Generation Attenuation (NGA) relations used for the 2008 USGS seismic hazard maps for Western United States (WUS) shall be utilized to determine the site-specific ground motion. When supported by data and analysis, other NGA relations, that were not used for the 2008 USGS maps, shall be permitted as additions or substitutions. No fewer than three NGA relations shall be utilized
2008 USGSBoore and Atkinson (2008)Campbell and Bozorgnia (2008)Chiou and Youngs (2008)
• Not an average velocity in upper 30 m
• Ratio of 30 m to shear wave travel time
What is Vs30?
(Stewart 2011)
• Not an average velocity in upper 30 m
• Ratio of 30 m to shear wave travel time
What is Vs30?
(Stewart 2011)
• Not an average velocity in upper 30 m
• Ratio of 30 m to shear wave travel time
• Emphasizes low Vs layers
What is Vs30?
(Stewart 2011)
Seismic Source Interpretation from PSHA Results
Deaggregation:Break the probabilistic “aggregation” back down to individual contributions based on magnitude and distance.
Provides:- Mean M,R: weighted average- Modal M,R: Greatest single contribution to hazard
Risk-Targeted MCER Probabilistic Response Spectrum
CRS = 0.941CR1 = 0.906
Deterministic MCER Response Spectra
Site-Specific MCER Response Spectrum
Design Response Spectrum
Site-Specific Response Spectra at Ground Surface
Sa (0.2s)= 1.42 Sa (1.0s)= 1.2Sa peak= 1.67 Sa (2.0s)= 0.740.9*Sapeak= 1.503 2*Sa(2s)= 1.48
SDS = 1.503 SD1 = 1.48SMS= 2.255 SM1= 2.22
SMSgen= 2.262 SM1gen= 1.6240.8*SMSgen= 1.810 0.8*SM1gen= 1.299
DESIGN ACCELERATION PARAMETERS
Site-specific MCE geometric mean (MCEG) PGA
PROB MCEG PGAThe probabilistic geometric mean PGA shall be taken as the geometric mean PGA with a 2% PE in 50 years
DETERMINISTIC MCEG PGACalculated as the largest 84th percentile geometric mean PGA for characteristic earthquakes on all known active faults. Minimum value 0.5 FPGA (FPGA at PGA=0.5g)
SITE-SPECIFIC MCEG PGALesser of probabilistic and deterministicMCEG PGA ≥ 0.80 PGAM
(Section 21.5)
SITE-SPECIFIC GROUND MOTION PROCEDURE
Site-specific Probabilistic MCER
(1% probability of collapse in 50 years)
METHOD 1CR * Sa (2% PE 50 year)
METHOD 2From iterative integration of a
site-specific hazard curve with a lognormal probability density function
representing the collapse fragilityCR = risk coefficient
(from maps)
T ≤ 0.2s CR = CRS
T ≥ 1.0s CR = CR1
0.2s < T < 1s Linear interp CRS and CR1
(i.e., probability of collapse as a function of Sa)
Collapse fragility with a) 10% Prob. of collapse; b) logarithmic std dev of
0.6
(Section 21.2.1)
PSHA Review…..
Risk is computed using a . Do you remember the concept of probabilistic seismic hazard analysis?
All possible magnitudes are considered - contribution of each is weighted by its probability of occurrence
All possible distances are considered - contribution of each is weighted by its probability of occurrence
All possible effects are considered - each weighted by its conditional probability of occurrence
Basic equation:
All sources andtheir rates ofrecurrence are considered
Performance-Based Earthquake Engineering
*1 1 1
[ * | , ] [ ] [ ]S M R
k ky i j ji j k
N N NP Y y P M P Rm mr r
Probabilistic framework
Pacific Earthquake Engineering Research Center (PEER) developed a probabilistic framework for considering the engineering effects from
EQ ground motions:
DV IMG DV DM dG DM EDP dG EDP IM d
Intensity measure,
IM
Engineering demand parameter
, EDP
Damage measure
, DM
Repair CostLives Lost
Down Time
Pile Deflection CrackingCollapse Potential
FSliq
Lateral Spread
SettlementStory Drift
PGAPGV
IACAV
Decision variable,
DV
Performance-Based Earthquake Engineering
dIMEDPdGEDPDMdGDMDVG IMDV
0
1
0.0P[D > 3.0 | PGA=0.3g]
P[D > 1.0 | PGA=0.3g]
P[D > 2.0 | PGA=0.3g]
0.3g
Example of Fragility curves
P[D > di | PGA]
3.0cm2.0cm
1.0cm
PGA
EDP = Displacement = DIM = PGA
Fragility Curves
The PEER performance-based framework incorporates seismic hazard curves and fragility curves. Convolving a fragility curve with a seismic hazard curve produces a single point on a new hazard curve!!
Seismic hazard
curve for IM (from
PSHA)
Fragility curve – EDP
given IM
Fragility curve – DM given EDP
Fragility curve – DV given DM
Risk curve – DV vs
DV
dIMEDPdGEDPDMdGDMDVG IMDV
Fragility Curves and Seismic Hazard Curves
Hazard curve
PGA
PGA
PGA
P[D>di| PGA]1.0
0.0
DPGA
D proportional to sum of thick
red lines
Fragility curve for D > 2.0cm
**
1
|N
EDP i IMi
P EDP EDP IM im
D
Fragility Curves and Seismic Hazard Curves
Hazard curve
PGA
IM
IM
1.0
0.0
DPGA
D proportional to sum of
probabilitiesFragility curve
D
D
Seismic hazard curve
for Displacemen
t
**
1
|N
EDP i IMi
P EDP EDP IM im
D
Fragility Curves and Seismic Hazard Curves
D=2.0cm
PGA
PGA
P[D>di| PGA]
Risk-targeted ground motions
(Luco 2009)
Risk-targeted ground motions
(Luco 2009)
Risk-targeted ground motions - Example
Summary of differences
ASCE 7-05 ASCE 7-10Name MCE MCER
Probabilistic GMs (objective)
Uniform hazard (2%-in-50 yr Pr. of Exc.)
Risk targeted (1%-in-50 yr Pr. of Collapse)
Deterministic GMs
1.5*median 84%-ile (approx. 1.8*median)
GM parameter Geometric mean, Sa Maximum direction, Sa
USGS web tool Java ground motion parameter calculator
Seismic design maps web application
Average SDS 0.73g 0.72gAverage SD1 0.38g 0.40g
(Luco 2009)
Contact Jorge F. Meneses, PhD, PE, GE, D.GE, F.ASCE
jmeneses@geiconsultants.com(760)795-1964
For further information
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